The present invention relates to a method and apparatus for molding a composite material article, more particularly such an article having opposite sides with differing surface configurations.
Techniques are well known for forming articles from composite materials whereby plies of pre-impregnated sheets, or prepregs, are stacked between a forming surface and gas-impermeable membrane. The article is formed by heating this assembly in an autoclave, while at the same time evacuating gases out to the volume enclosed by the membrane.
U.S. Pat. No. 4,562,033 describes a method for manufacturing articles from a composite material wherein a stack of prepregs are placed on a former having a forming surface and then are covered by a microporous film as well as a gas-impermeable membrane. A vacuum is drawn between the membrane and the former and the assembly is heated to first de-gas the resin of the composite material, and then to subsequently cure the composite material. The curing takes place at atmospheric pressure. This technique has been found to be suited for composite material parts in which the opposite sides have identical configurations.
U.S. Pat. No. 4,683,018 describes a method and apparatus for shaping composite material parts. Initially, the prepreg layers are separated by release films and the assembly is subjected to partial curing while interposed between two shaped formers. Pressure is exerted onto the stack of layers by a press acting on one of the formers. Following the partial curing, the layers are removed from between the formers, the release layers removed from the stack and the stack is subjected to a final curing operation by placing it between a former and an impermeable membrane, placing the assembly in an autoclave and curing the resin. Again, this technique has been found to be suited for the manufacture of composite material articles having opposite sides with the same surface configuration.
French Patents 2,526,011 and 2,459,118 also illustrate typical examples of the known techniques wherein a stack of prepregs is interposed between a forming surface and a flexible membrane which exerts a compressing force on the prepregs during the curing of the resin.
The techniques disclosed by the above patents form composite material parts by curing the-stack of prepregs in an autoclave in which the pressure compacting the laminate is isostatic. One of the drawbacks of these known techniques is that the use of additional layers, such as environmental products, plastic films, glass felts and other fabrics between the stack of prepregs and the impermeable membrane allows the accurate forming of only the surface which is in contact with the forming surface of the former. Such techniques are suitable for composite material parts which have opposite surfaces of the same configuration, in which the surfaces are separated by a generally constant thickness of the composite material. Accordingly, these techniques cannot be utilized to manufacture composite material articles which have opposite sides of differing surface configurations, such as fan blades.
Other techniques involve the stacking of the prepreg plies in a mold and compressing the stack between mold portions by a press, or the like in which curing takes place in the press. This technique transmits axial stresses to the stack of prepregs, thereby resulting in-a composite material article which is less than completely satisfactory.